PROJECT SUMMARY The genetic study of severe human congenital cerebrovascular anomalies can shed insight into mechanisms of normal vascular development and identify targets for therapeutic intervention. Vein of Galen aneurysmal malformations (VOGMs) are the most common and severe of pediatric brain arterio-venous malformations (AVMs). Significant gaps in our understanding of the molecular pathogenesis of VOGMs impede the development of improved diagnostic and therapeutic measures. Locus heterogeneity and the sporadic nature of VOGM cases have constituted fundamental obstacles to VOGM gene discovery. We recently applied whole exome sequencing (WES) to overcome these obstacles and identified de novo and inherited gene mutations that account for ~30% of sporadic VOGM cases (Duran et al., Neuron, 2019). These included a genome-wide significant burden of rare, damaging mutations in EPHB4 (EphB4), a critical regulator of arterio-venous specification also mutated in the familial AVM syndrome, capillary malformation (CM)-AVM type II (CM-AVM2). We also discovered new mutations in other genes that function in the same Ephrin signaling interactome, including RASA1 (also mutated in CM-AVM1). We further demonstrated that EphB4 exists in a physical complex with RASA1, and have now solved the first multi-domain crystal structure of RASA1. Nonetheless, most VOGM cases remain genetically unsolved, and the molecular mechanisms of VOGM-associated mutations are poorly understood. To address these knowledge gaps, we propose a functional genomics approach to discover and mechanistically elucidate VOGM-associated mutations with atomic-level resolution. We hypothesize WES will identify novel VOGM genes and mutations, including mosaic and somatic ?second-hit? mutations, which disrupt the regulated activity of an EphB4-RASA1 signaling complex essential for arterio-venous development. Based on our successful experience in identifying structural brain disorder genes over the past several years, Aim 1 will ascertain additional VOGM case-parent trios and perform WES on our growing cohort (already the largest in the world) to discover novel de novo and transmitted germline VOGM gene mutations, mosaic variants, and somatic mutations in lesional tissue. In Aim 2, we will determine the structural and functional impact of VOGM mutations using biochemical, biophysical, structural biology and cell biology techniques, with validation experiments in autopsied VOGM tissue, and in skin biopsies of VOGM patients with associated cutaneous vascular malformations. Successful completion of these Aims will increase our understanding of human cerebrovascular development and VOGM pathophysiology. These advances will improve disease management and genetic counseling, and will stimulate development of targeted therapeutics for VOGMs that may be broadly relevant for other vascular lesions, including AVMs and intracranial aneurysms.